The present invention generally relates to communications systems, and more particularly to methods and apparatus for transmultiplexing a multi-channel communication signal in a multi-channel transceiver.
There are numerous advantages to implementing a communications system using digital techniques. For example, there are enhanced system capacity, and reduced noise, hardware and power consumption. In communications systems having a multi-channel communication signal, multi-channel transceivers with transmultiplexers are utilized to recover individual communication channels from the multi-channel communication signal. Such a multi-channel transceiver 100 having a transmultiplexer 102 is shown in FIG. 1 according to the prior art.
The multi-channel transceiver 100 shown in FIG. 1 is configured as a receiver for illustrative purposes. However, it should be appreciated that operated in reverse, this multi-channel transceiver 100 operates to process multiple channels for transmission as a multi-channel radio frequency (RF) signal. As a receiver, the transceiver 100 receives multi-channel RF signals with an antenna 104, transducer 106, bandpass filter 108, mixer 110, intermediate frequency (IF) filter 112, baseband mixer 114, filter 116, analog-to-digital (A/D) converter 118 and mixer 120.
The mixer 120 mixes the digitized signal output 122 of the A/D converter 118 with a complex sinusoid of the form   ⅇ                    j2π        ⁢                  xe2x80x83                ⁢                  kF          0                            F        s              ,  
where Fs is the A/D sample frequency, F0 is the frequency shift, and k is the time index. This frequency shift is often used to translate a selected frequency channel such that it is centered at 0 Hz. Therefore, we refer to this operation as channel centering. This channel centering operation creates a digitized and centered multi-channel signal 124, which is operatively coupled to the transmultiplexer 102 by commutator 126. The transmultiplexer 102 converts the digitized and centered multi-channel signal 124 to a baseband signal using a fast Fourier transformer 128 operating on the outputs of polyphase filters 130, and equalizes and/or matched filters each of the down-converted channels of the fast Fourier transformer 128 using equalizers 132 and/or matched filters 134, respectively. The resulting outputs 136 of the transmultiplexer 102 are multiple communications channels separated from the digitized and centered multi-channel signal 124 originally presented to the inputs 138 of the transmultiplexer 102.
As may be appreciated from the foregoing description of the prior art, the transmultiplexer 102 requires channel centering to avoid additional computations caused by down converting both the positive and negative images of the channels in the real-valued input signal. The channel centering operations result in complex valued signals that are presented to the transmultiplexer for channel signal conversion, thereby increasing computational requirements as compared to a transmultiplexer conversion of real value inputs. Furthermore, the polyphase filtering, equalization and matched filtering of the prior art transmultiplexer does not utilize fast Fourier transform (FFT) filtering and fast convolution to improve computational efficiency.
In view of the foregoing, is should be appreciated that it would be desirable to provide transmultiplexing methods and apparatus for use with digital transceivers that reduce channel duplication. In addition, it should be appreciated that it would be desirable to provide transmultiplexing methods and apparatus with improved computational efficiency. Furthermore, additional desirable features will become apparent to one skilled in the art from the following detailed description of a preferred exemplary embodiment and appended claims.